In optical fibers used in optical communications systems, orthogonal polarization modes have different group delays; known as differential group delay (DGD). This causes the polarization mode dispersion (PMD) phenomenon, i.e., a spreading of the pulses propagating along the fibers Where long optical fiber links are involved, overall PMD may be sufficient to cause increased bit error rate, thus limiting the transmission rate or maximum transmission path length. This is particularly problematical at higher bit rates. As a variable or quantity characterizing the said phenomenon, the PMD value of a device is defined as either the mean value or the root-mean-square (RMS) value of DGD (the DGD of a given device is a random variable that varies over both wavelength and time).
As explained in commonly-owned U.S. Pat. No. 6,724,469 (Leblanc), in optical communication systems, an unacceptable overall polarization mode dispersion (PMD) level for a particular long optical fiber may be caused by one or more short sections of the overall optical fiber link. Where, for example, a network service provider wishes to increase the bitrate carried by an installed optical fiber link, say up to 40 Gb/s, it is important to be able to obtain a distributed measurement of PMD, i.e., obtain the PMD information against distance along the fiber, and locate the singularly bad fiber section(s) so that they can be replaced—rather than replace the whole cable.
It is known to use a so-called polarization optical time domain reflectometer (POTDR) to try to locate such sections. Whereas conventional optical time domain reflectometers (OTDRs) measure only the intensity of backscattered light to determine variation of attenuation along the length of a transmission path, e.g., an installed optical fiber, POTDRs utilize the fact that the backscattered light also exhibits polarization dependency to monitor polarization dependent characteristics of the transmission path. Basically, a POTDR is an OTDR that is sensitive to the state of polarization (SOP) of the backscattered signal. Thus, the simplest POTDR comprises an OTDR having a polarizer between its output and the fiber-under-test (FUT) and an analyzer in the return path, between its photodetector and the FUT. (It should be appreciated that, although a typical optical transmission path will comprise mostly optical fiber, there will often be other components, such as couplers, connectors, etc., in the path. For convenience of description, however, such other components will be ignored, it being understood, however, that the term “FUT” used herein will embrace both an optical fiber and the overall transmission path according to context.)
Generally, such polarization optical time domain reflectometers can be grouped into two classes or types. Examples of the first type of POTDR are disclosed in the following documents:                F. Corsi, A. Galtarossa, L. Palmieri, “Beat Length Characterization Based on Backscattering Analysis in Randomly Perturbed Single-Mode Fibers,” Journal of Lightwave Technology, Vol. 17, No. 7, July 1999.        A. Galtarossa, L. Palmieri, A. Pizzinat, M. Schiano, T. Tambosso, “Measurement of Local Beat Length and Differential Group Delay in Installed Single-Mode Fibers”, “Journal of Lightwave Technology, Vol. 18, No. 10, October 2000. (N.B. only total PMD from end-to-end is measured for comparison, not cumulative PMD vs z.).        A. Galtarossa, L. Palmieri, M. Schiano, T. Tambosso, “Measurement of Beat Length and Perturbation Length in Long Single-Mode Fibers,” Optics Letters, Vol. 25, No. 6, Mar. 15, 2000.        B. Huttner, “Distributed PMD measurement with a polarization-OTDR in optical fibers”, Journal of Lightwave Technology, Vol. 17, pp. 1843-1948, March 1999.        U.S. Pat. No. 6,946,646 (Chen et al.)        US published patent application number 2004/0046955, Fayolle et al.        
The first type of POTDR basically measures local birefringence (1/beat-length) as a function of distance z along the fiber, or, in other words, distributed birefringence. Referring to the simple and well-known example of a retardation waveplate, birefringence is the retardation (phase difference) per unit length between the “slow” and “fast” axes. In other words, the retardation is the birefringence times the thickness of the waveplate. This is not a PMD measurement, though that is a common misconception. First, in a simplified picture, DGD(z) is the derivative, as a function of optical frequency (wavelength), of the overall retardation of the fiber section extending from 0 to z. Second, a long fiber behaves as a concatenation of a large number of elementary “waveplates” for which the orientation of the fast and slow axes, as well as the retardation per unit length, vary randomly as a function of distance z.
Accordingly, DGD(z) is the result of a complicated integral over all that lies upstream that exhibits random birefringence and random orientation of the birefringence axis as a function of z, whereas birefringence is the retardation per unit length at some given location. Additionally, as mentioned above, the derivative, as a function of optical frequency, of such integral must be applied in order to obtain DGD as per its definition.
A general limitation of all the techniques of this first type, therefore, is that they do not provide a direct, reliable, valid in all cases and quantitative measurement of PMD with respect to distance along the optical fiber. Instead, they measure local birefringence (or beat-length) and/or one or more related parameters and infer the PMD from them based notably on assumptions about the fiber characteristics and specific models of the birefringence. For instance, they generally assume a relationship between PMD and local values of the birefringence and so-called coupling-length (or perturbation-length), which does not necessarily stand locally even when it stands in average.
As an example, such techniques assume that fibers exhibit exclusively “linear” birefringence. If circular birefringence is indeed present, it is “missed” or not seen, because of the properties of a round trip through the fiber (OTDR technique). Notably, twisted fibers like modern spun fibers already require some special models, which implies that an instrument must know in advance the type and characteristics of the FUT, which is unacceptable for a commercial instrument.
As a second example, the birefringence and other parameters must be measured accurately throughout the length, even in sections where the local characteristics of the fiber do not satisfy the assumed models and conditions; otherwise, the inferred PMD of such sections, which is an integral over some long length, can be largely misestimated, even qualitatively speaking. In practice, although they can measure birefringence quantitatively (cf. F. Corsa et al. supra), or statistically screen high birefringence sections (Chen et al. supra), or obtain qualitative and relative estimates of the PMD of short sections provided that one accept frequently occurring exceptions (Leblanc, Huttner, supra), POTDR techniques of this first type cannot reliably and quantitatively measure PMD, particularly of unknown, mixed installed fibers in the field. Furthermore, they are incapable of inferring, even approximately, the overall PMD of a long length of fiber, such as for example 10 kilometers.
Fayolle et al. (supra) claim to disclose a technique that is “genuinely quantitative, at least over a given range of polarization mode dispersion”. However, this technique also suffers from the fundamental limitations associated with this type, as mentioned above. In fact, while their use of two SOPs (45° apart) with two trace variances might yield a modest improvement over the similar POTDRs of the first type (e.g., Chen at al.'s, whose VOS is essentially the same as Fayolle et al.'s trace variance), perhaps by a factor of √{square root over (2)}, it will not lead to a truly quantitative measurement of the PMD with respect to distance along the FUT with an acceptable degree of accuracy. It measures a parameter that is well-known to be related or correlated with beat-length (birefringence), but not representative of the PMD coefficient. Indeed, even the simulation results disclosed in Fayolle et al.'s specification indicate an uncertainty margin of 200 percent.
It is desirable to be able to obtain direct, quantitative measurements of PMD, i.e., to measure the actual cumulative PMD at discrete positions along the optical fiber, as if the fiber were terminated at each of a series of positions along its length and a classical end-to-end PMD measurement made. This is desirable because the parameter that determines pulse-spreading is PMD, not birefringence. If one knows the actual PMD value of a communications link one can determine, accurately, the bit error rate or outage probability (probability that the communication will fail over a period of time), or the power penalty (how much more power must be launched to maintain the same bit error rate as if there were no PMD).
(In this specification, the term “cumulative PMD” is used to distinguish from the overall PMD that is traditionally measured from end to end. Because PMD is not a localized quantity, PMD(z) is an integral from 0 to z, bearing resemblance to a cumulative probability rather than the probability distribution.)
The second type of known POTDR is dedicated specifically to PMD measurement. This type does not suffer from the above-mentioned fundamental limitations of the first type of POTDR and so represents a significant improvement over them, at least in terms of PMD measurement. It uses the relationship between POTDR traces obtained at two or more closely-spaced wavelengths in order to measure PMD directly at a particular distance z, i.e. cumulative PMD, without any assumption about the birefringence characteristics of the fibers, nor any need for an explicit or implicit integral over length, no missed sections, no problem with spun fibers, and so on. Even a circularly birefringent fiber or a section of polarization-maintaining fiber (PMF) is measured correctly. In contrast to implementations of the first type, there is no need to invoke assumptions and complicated models in order to qualitatively infer PMD.
Thus, measurement of cumulative PMD as a function of distance z along the fiber, and its slope, as allowed by a POTDR of this second type, facilitates reliable identification and quantitative characterization of those singular, relatively-short sections where the slope of the PMD vs. distance is large over some distance, thus accounting for almost all the PMD of the link, the rest contributing a much smaller percentage of the total PMD.
Most known POTDR techniques of this second type rely upon there being a deterministic relationship between the OTDR traces obtained with a small number of specific input-SOP and output polarization axes, as disclosed, for example, in U.S. Pat. No. 6,229,599 (Galtarossa), an article by H. Sunnerud, B-E. Olsson, P. A. Andrekson, “Measurement of Polarization Mode Dispersion Accumulation along Installed Optical Fibers”, I.E.E.E. Photonics Technology Letters, Vol. 11, No. 7, July 1999 and an article by H. Sunnerud, B-E. Olsson, M. Karlsson, P. A. Andrekson and J. Bretnel entitled “Polarization-Mode Dispersion Measurements Along Installed Optical Fibers Using Gated Backscattered Light and a Polarimeter”, Journal of Lightwave Technology, Vol. 18, No. 7, July 2000. This requires the FUT to be spatially stable throughout the time period over which all the traces are measured. Unfortunately, such stability cannot be assured, especially where an installed fiber is being measured.
In addition, known techniques of the second type require the use of short pulses, “short” meaning shorter than the beat length and coupling length of any section of the FUT. In order for them to measure high PMD in fibers properly, without being limited to fibers of very large beat length (which often will have low PMD), they must use OTDR optical pulse widths of less than 5 to 10 ns at maximum. Unfortunately, practical OTDRs do not have a useful dynamic range with such short pulses. On the other hand, if a long light pulse is used, only fibers having long beat lengths can be measured, which limits these techniques, overall, to measurement of short distances and/or with long measurement times, or to fibers with large beat length (typically small PMD coefficient). Hence, although it might be possible, using known techniques and meeting the above-mentioned requirements, to make a reasonably successful measurement, at present their scope of application and performance would be insufficient for commercially-viable, stand-alone instrument.
In addition, the use of short pulses exacerbates signal-to-noise ratio (SNR) problems due to the so-called coherence noise that superimposes on OTDR traces and is large when short pulses are used. It is due to the fact that the power of the backscattered light is not exactly the sum of powers emanating from each element (dz) of the fiber. With a coherent source such as a narrowband laser, as used in POTDR applications, there is interference between the different backscattering sources. This interference or coherence noise that is superimposed on the ideal trace (sum of powers) is inversely proportional to both the pulse width (or duration) and the laser linewidth. It can be decreased by increasing the equivalent laser linewidth, i.e. the intrinsic laser linewidth as such, or, possibly, by using “dithering” or averaging traces over wavelength, but this reduces the maximum measurable PMD and hence may also limit the maximum length that can be measured, since PMD increases with increasing length. Roughly speaking, the condition is PMD*Linewidth<1; otherwise the useful POTDR signal is “washed out” by depolarization.
Accordingly, known POTDR techniques suffer from the limitation that they do not measure, quantitatively and accurately, cumulative PMD at specific distances along a FUT, especially a long optical fiber of the kind now being used in optical communications systems, with a satisfactory dynamic range (long pulses) and without stringent requirements regarding the stability of the FUT.